Attention has been paid to employment of an excimer laser light unit as a light source for an equipment for projecting and exposing an image in a reduced scale (hereinafter referred to as a stepper) wherein the stepper is installed for producing semiconductor devices. The reason for employment of the excimer laser light unit consists in that many significant advantages can be expected from the employment of the excimer laser light unit based on the fact that an excimer laser light has a short wavelength (e.g., about 248.4 nm in the case of a KrF laser light), one of the foregoing advantages being the presence of a possibility that a limit of light exposure is extensively set to the range shorter than 0.5 micron, other one being such that the unit has a deep focus compared with the conventional mercury lamp including a g line and an i line on the assumption that they have a same resolution, respectively, another one being such that the unit has a small number of lens apertures (NA), another one being such that the exposure range can be enlarged and further another one being such that the unit can generate a large magnitude of power.
Since the excimer laser light has a short wavelength of 248.4 nm, only a quartz, CaF.sub.2, MaF.sub.2 or the like material are available as a material employable for permitting the laser light having the foregoing wavelength to permeate therethrough. In this connection, it should be added that the quartz only can be employed as a raw material for lenses due to various restrictions associated with optical uniformity, working accuracy and others. This makes it very difficult to design a lens employable for projecting an image in a reduced scale with its chromatic aberration corrected properly. For the reason, in the case where the excimer laser light unit is used as a light source for the stepper, there is a need of restricting the use of a laser light outputted from the excimer laser light unit within the narrow-band range to such an extent that the chromatic aberration may be neglected and moreover stably controlling the wavelength of an output laser light within the narrow-band range with a high accuracy.
A monitor etalon has been heretofore used for measuring or detecting the wavelength of an output light from an excimer laser light unit adapted to oscillate within the narrow-band range. The monitor etalon is constructed by using an air gap type etalon comprising a pair of partially refractive mirrors arranged opposite to each other with a predetermined gap therebetween. Here, the wavelength of a light to permeate through the air gap type etalon is expressed by the following equation. EQU m.lambda.2n d cos .theta.
where m designates a certain integral, d designates a distance between the partially refractive mirrors constituting the etalon, n designates a refractive index between the partially refractive mirrors and .theta. designates an angle defined by a normal line of the etalon and an optical axis of the incident light.
As is apparent from the equation, the angle .theta. varies corresponding to variation of the wavelength, provided that n, d and m are constant, respectively. The monitor etalon is used to detect the wavelength of a light to be detected by utilizing the characteristic nature as mentioned above. With respect to the above-described monitor etalon, it has been found that as the pressure within the air gap and the environmental temperature vary, the angle .theta. varies correspondingly even through the wavelength is kept constant. Thus, in the case where the monitor etalon is used, the wavelength is detected while the pressure within the air gap and the environmental temperature are controlled such that they are kept constant.
In practice, however, it is difficult to control the pressure within the air gap and the environmental temperature with a high accuracy. This makes it impossible to detect the absolute wavelength of a light with a sufficiently high accuracy.
To obviate the above problem, a proposal has been made with respect to an apparatus for detecting the absolute wavelength of a light to be detected by allowing a reference light of which wavelength is previously known to be inputted into the monitor etalon together with the light to be detected and then detecting a relative wavelength of the light to be detected relative to the reference light. However, in the case where a light source for the reference light and a light source for the light to be detected are different from each other in respect of their characteristic nature, e.g., in the case where a light source for the reference light is a mercury lamp which may be considered as a plane light source and a light source for the light to be detected is an excimer laser light unit, it is difficult for the reference light and the light to be detected to be introduced into the monitor etalon with a sufficiently high intensity, causing a ratio of S/N to be degraded. This makes it impossible to detect an absolute wavelength of the light to be detected with a high accuracy.
In the case where the apparatus for detecting the absolute wavelength of a light to be detected is constructed by allowing a reference light of which wavelength is previously known to be introduced into a monitor etalon together with the light to be detected to detect a relative wavelength of the light to be detected relative to the reference light and then detect an absolute wavelength of the light to be detected with reference to the detected relative wavelength in the above-described manner, when a light source for the reference light and a light source for the light to be detected are different from each other in respect of their characteristic nature, it becomes difficult that the reference light and the light to be detected are introduced into the monitor etalon with a sufficiently high intensity, respectively, resulting in the absolute wavelength of the light to be detected failing to be detected with a high accuracy.
The present invention has been made with the foregoing background in mind.
Therefore, an object of the present invention is to provide a wavelength detecting apparatus which assures that a reference light and a light to be detected can be introduced thereinto with a sufficiently high intensity even though a light source for the reference light and a light source for the light to be detected are different from each other in respect of their characteristic nature, whereby a wavelength of the light to be detected can be detected with a high accuracy.